Rare earth catalyst system: lanthanide phosphonate/tri-i-butyl aluminum (Nd(P-204)(3)/Al(i-Bu)(3)) has been found for the first time as a novel catalyst for the polymerization of n-hexyl isocyanate (HNCO). Nd(P-204)(3) and Nd(P-507)(3) are the commercial names of neodymium 2-ethylhexyl phosphonates, their formulas are shown in table 1. The catalyst can be prepared easily by mixing Nd(P-204)(3) and Al(i-Bu) (3). The effects of catalyst, solvent, reaction temperature and time on the polymerization of HNCO were studied. The obtained poly (n-hexyl isocyanate) (PHNCO) was characterized by GPC, FT-IR, H-1-NMR and TGA. The resulting PHNCO had molecular weight (Mn=39.6 x 10(4), Mv=67.2 x 10(4)), molecular weight distribution (MWD=2.44) and yield (82.7%) under the moderate reaction conditions: catalyst concentration [Nd]= 6.65 x 10(-2)mol/ L, Al/Nd = 10 molar ratio, [HNCO]/[Nd]= 100 molar ratio, at - 10degreesC for 10h in bulk. Relatively high reaction temperature (- 10degreesC) is the most distinct virtue. The IR and NMR analyses show that the polymer obtained is not polyether but polyisocyanate.